The District Heating systems (DH) are distribution systems of thermal energy, normally in the way of hot water that is supplied to great areas of population, so the users of these systems use them in the heating of their buildings and in the preparation of the hot tap water that is consumed in the buildings. The kinds of power stations that supply the thermal energy to the DH systems are as wide and varied as could be imagined: big boilers rooms, geothermal stations, solar stations,..., but mainly and due to the high energetic efficiency, the power stations which have been widely spread in the last years are Combined Heat and Power plants (CHP). In this article, it is presented a brief description of the DH systems in the first place and then, it is shown the main advantages that the centralized thermal production has, compared with the individual systems, and the kinds of CHP Plants more extensively used in the DH systems are described. Furthermore, this article focus on the different aspects that contribute to optimize the electric and thermal production in the CHP plants, as the operation way, thermal storage or the requirements that the DH systems have to achieve.